An oyster spat is a squiggle less than an inch long, looking for someplace to stick.

Once attached to something hard, the tiny larva creates its own shell, other spats attach and an oyster shoal forms. The shoals do a lot more than just make oysters. They stabilize banks from erosion and allow marsh to grow.

And that’s why coastal conservationists are now building castles in the sand.

The oyster castle, a tiered and turreted concrete block, is one of a number of oyster structures including mats, baskets, poles and blocks that are being set along the Southeast coast to supplement the more established loose-shell or bagged-shell restorations done to increase oyster harvest.

There’s just one little sticking point: The castles and other structures aren’t open to harvest, at least so far.

But they are showing some magic.

The U.S. Army Corps of Engineers, Charleston District, planted its first castles last year off the Intracoastal Waterway near Isle of Palms, to shore up the bank of a dredging-material disposal pit.

In just one year the castles are nearly covered with oysters, and the marsh is filling in behind them.

“Protecting bank, building bank. We’ve got spartina (grass) growing behind it. It’s relatively inexpensive compared to (piling) rocks. And we’ve got erosion control. Everything,” said David Warren, Army Corps project manager.

The Nature Conservancy in 2009 planted the first pallet of castles in the state in Cape Romain. The environmental group now has about 2,000 square feet overall, said Joy Brown, of the Charleston chapter.

The efforts mirror projects underway across the region, including in Georgia, Florida, Alabama, Louisiana and Texas. In Georgia, the conservancy has taken part in projects using oyster baskets, bags, oak-limb bundles and bamboo “spat sticks” in locations that include Sapelo Island.

The Army Corps also has a project in Chesapeake Bay.

Oyster beds are considered to be in precipitous decline worldwide, with some 85 percent of the beds lost in the past century.

In the region, beds have been lost to everything from storms to boat wakes. The S.C. Department of Health and Environmental Control closed about one-third of the state’s acreage to harvest in two decades because of pollution.

Since the 1980s, the Department of Natural Resource has focused on supplementing harvest shoals, adding an average of 5 to 6 acres per year. The state now has about 4,500 acres.

“We can’t say whether we have more or less than we used to, but we certainly haven’t had a significant loss. We have an awful lot of oysters,” said Nancy Hadley, DNR shellfish management section manager.

But loose- and bagged-shell bed restorations — the types that can be harvested — are a pick-your-spot effort that can be destroyed by boat wakes or waves. The state sites are carefully selected, but only about 70 percent succeed, Hadley said.

Wakes and waves are just what the castles and other structures protect against. But because oysters grow mostly intertidal, the castles tend to be limited in how many oysters they produce. And chipping oysters off the concrete would undermine that protection, so they are off limits.

“There’s no way oyster castles are ever going to become an oyster reef. It’s more like a sea wall,” Hadley said.

The conservancy’s Charleston chapter is now looking to expand its program, combining structure types to create bigger shoals that potentially could be at least partly harvested.

“We’re working towards that. We’re not there yet,” Brown said.

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